1. Field of the Invention
This invention is related to methods for acquiring and processing nuclear magnetic resonance (NMR) measurements for determination of longitudinal and transverse relaxation times T1, and T2. Specifically, the invention deals with use of an expert system downhole for acquiring and evaluating NMR measurements contemporaneous with the drilling of wells and with use of a downlink communication from the surface for modifying the parameters of the downhole acquisition system.
2. Description of the Related Art
Nuclear magnetic resonance is used in the oil industry, among others, and particularly in certain oil well logging tools. NMR instruments may be used for determining, among other things, the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space of earth formations. Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluids are described, for example, in “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,” M. N. Miller et al., Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990. Further description is provided in U.S. Pat. No. 5,585,720, of Carl M. Edwards, issued Dec. 17, 1996 and having the same assignee as the present application, entitled “Signal Processing Method For Multiexponentially Decaying Signals And Applications To Nuclear Magnetic Resonance Well Logging Tools.” The disclosure of that patent is incorporated herein by reference.
Deriving accurate transverse relaxation time T2 relaxation spectra from nuclear magnetic resonance (NMR) data from logging subterranean formations, or from cores from such formations, is critical to determining total and effective porosities, irreducible water saturations, and permeabilities of the formations. U.S. Pat. No. 6,069,477 to Chen et al having the same assignee as the present application discusses the constituents of a fluid saturated rock and various porosities of interest. Referring to FIG. 1, the solid portion of the rock is made up of two components, the rock matrix and dry clay. The total porosity as measured by a density logging tool is the difference between the total volume and the solid portion. The total porosity includes clay-bound water, capillary bound water, movable water and hydrocarbons. The effective porosity, a quantity of interest to production engineers is the sum of the last three components and does not include the clay bound water. Accurate spectra are also essential to estimate T2 cutoff values and to obtain coefficients for the film model or Spectral Bulk Volume Irreducible (SBVI) model. Effective porosities are typically summations of partial porosities; however, distortion of partial porosity distributions has been commonly observed for a variety of reasons. These reasons include poor signal-to-noise ratio (SNR), and poor resolution in the time domain of the NMR data.
The most common NMR log acquisition and core measurement method employs T2 measurements using CPMG (Carr, Purcell, Meiboom and Gill) sequence, as taught by Meiboom and Gill in “Modified Spin-Echo Method for Measuring Nuclear Relaxation Time,” Rev. Sci. Instrum. 1958, 29, pp. 688–691. In this method, the echo data in any given echo train are collected at a fixed time interval, the interecho time (TE). Usually, a few hundred to a few thousand echoes are acquired to sample relaxation decay. However, for determination of CBW, echo sequences of as few as ten have been used.
There are numerous examples of wireline NMR logging tools used for obtaining information about earth formations and fluids after a wellbore has been drilled. The logging tools are lowered into the borehole and NMR signals are obtained using different configurations of magnets, transmitter coils and receiver coils. Rig time is expensive, so that the general objective in wireline logging is to obtain interpretable data within as short a time as possible. Depending upon the reservoir, different radio frequency (RF) pulsing schemes for generating RF fields in the formation have been used. The most commonly used pulsing schemes are variations of the CPMG sequence. The parameters that may be varied are the wait time, the number of pulses within a CPMG sequence, and the time interval between the pulses. Long wait times are needed for proper evaluation of the long relaxation times of gas reservoirs while short wait times and/or short pulse spacings are used for evaluating clay bound water (CBW). For example, co-pending U.S. patent application Ser. No. 09/396,286 (now U.S. Pat. No. 6,331,775) of Thern et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discusses the use of a dual wait time acquisition for determination of gas saturation in a formation. U.S. Pat. No. 5,023,551 to Kleinberg et al discusses the use of CPMG sequences in well logging. U.S. Pat. No. 6,069,477 to Chen et al, the contents of which are fully incorporated herein by reference, teaches the use of pulse sequences with different pulse spacings to determine CBW. Phase alternated pairs (PAPs) of sequences are commonly acquired to reduce the effects of ringing.
Tool vibration is usually not a problem in wireline logging, so that data may be acquired using continuous pulsing while the logging tool is being pulled up the borehole. In many instances, other logs may already have been run before the NMR measurements are made, so that some preliminary evaluation of the subsurface formations may already exist. This makes it possible to use predefined pulse sequences optimized for specific evaluation objectives.
The commonly used seven conductor wireline is not a serious limitation to two-way communication from the surface to the logging tool. This makes it possible to process data uphole with little or no downhole processing and to send instructions downhole to the logging tool to modify the acquisition scheme based on the surface processing.
In contrast, measurements made with a drilling assembly in the wellbore have several problems. First of all, there is little prior information available about the actual subsurface formations except that inferred from surface seismic data. As would be known to those versed in the art, the resolution of such seismic data is of the order of several meters to tens of meters. This makes it difficult, if not impossible, to base an acquisition scheme on the basis of expected properties of formations.
Secondly, when the drilling assembly is in a borehole, data communication capability is in most cases severely limited. Telemetry is accomplished either by sending acoustic pulses through the mud or through the drillstring. The data rate with mud pulsing is limited to a few bits per second and communication through the drillstring becomes a serious problem when the drillbit is being operated due to the vibration and noise produced. This makes it impossible to evaluate acquired data at the surface and to modify the acquisition scheme based on this evaluation.
A third problem arises from the nature of NMR data itself. The sensitive volume of commonly used logging tools is no more than a few millimeters in thickness. The RF frequency is tuned to operate at the Larmor frequency corresponding to the static magnetic field in the sensitive volume, so that any transversal motion of the tool during drilling will mean that the RF pulses have a frequency corresponding to a region that has not been pre-polarized by the static magnetic field. This results in a severe degradation of the data. U.S. Pat. No. 5,705,927 issued to Kleinberg discloses making the length of each CPMG sequence small, e.g. 10 ms, so that the drill collar cannot be displaced by a significant fraction of the vertical or radial extent of the sensitive region during a CPMG pulse sequence. However using such short sequences and short wait times only gives an indication of the bound fluid volume and gives no indication of the total fluid volume.
There is a need for an apparatus and method of obtaining NMR measurements while a wellbore is being drilled that is able to modify the acquisition parameters with a minimum of communication with the surface. Such an invention should preferably be able to adjust the acquisition depending upon actual downhole conditions. The method should preferably be robust in the presence of vibration of the logging tool. The present method satisfies this need.